As waste enters the septic tank, bacteria will actively break down the waste, which leaves water behind that can percolate down in the ground. Your septic system depends on bacteria to do its job well. The bacteria lives down in the drain field and in the septic tank.
All waste which flows in your septic tank is called effluent. As bacteria breaks down organic materials, it will form a layer of sludge down on the bottom of the tank. The sludge is made up of inorganic solids, as well as the bacteria byproducts.
While sludge will fall to the bottom, there is a layer of scum that floats up to the top. This scum is made of grease, fat, and oils. Your septic tank works just as a settling pond.
Each septic system will have a filter which stops the majority of solids from entering an outlet pipe, leading to the drain field. The drain field is a big surface in which bacteria feeds on the effluent, and as the treated water will seep into the ground. Pipes that carry the treated water have holes for the water to effectively drain while the surrounding gravel around the pipes allow for oxygen to reach the bacteria.
Along with the assistance of bacteria, the waste will be decomposed; the water will be returned to the groundwater.